Random pulse width modulation (RPWM) is recognized as a desirable technique to reduce both electromagnetic and acoustic noise emissions from pulse width modulation (PWM) inverters. RPWM is generally characterized by random variations of the switching frequency. The random variations of the frequency alleviate undesirable characteristics in PWM electronic power converters. Specifically, the fundamental AC component harmonics remain unchanged. However, the spectral power, measured in Watts, is converted to continuous power density, measured in Watts per Hertz, instead of being concentrated in discrete harmonics. The power spectra of the output voltage and current from a RPWM power converter emulate the spectrum of white noise. Consequently, spurious phenomena are significantly mitigated.
Additionally, conventional variable-delay random pulse width modulation (VD-RPWM) may also be used for various applications to further alleviate undesirable characteristics. In fact, the variable-delay random PWM technique provides a number of significant advantages over other RPWM techniques.
Known prior art systems have demonstrated the excellent EMC performance of true random switching frequency modulation techniques where both the sampling and PWM periods are synchronized. However, these RSF systems suffer from a significant disadvantage, namely the maximum code size is limited by the minimum sample period. Furthermore, the random sample rate places a constraint on the minimum sample period based upon the required time to execute the application code. For complicated motor control algorithms, the length of code may not allow sufficiently high switching frequency to achieve good spectral spreading.
Fixed sample rate techniques, on the other hand, allow optimal use of the processor computational capability. For example, random zero vector, random center displacement, and random lead-lag techniques all maintain synchronous sample and PWM period, but suffer some form of limitation. For example, random zero vector and random center displacement lose effectiveness at high modulation indexes. Random lead-lag does not offer suitable performance with respect to reducing acoustic/EMI emissions and, further, suffers an increased current ripple. Additionally, both random lead-lag and random center displacement introduce an error in the fundamental component of current due to a per-cycle average value of the switching ripple.
The VD-RPWM technique allows a fixed sample rate for optimal usage of processor computational power, while providing quasi-random PWM output for good spectral spreading. However, conventional VD-RPWM suffers from disadvantages when operated at high fundamental frequencies. For example, using a 4-pole induction machine with a maximum speed of 14 krpm, the highest fundamental electrical frequency is 467 Hz. In this situation, using a 12 kHz sample rate, conventional VD-RPWM techniques provide satisfactory control. On the other hand, when used with induction machines having eight or more poles, the highest fundamental electrical frequency may exceed 800 Hz. In these cases, the delay introduced by VD-RPWM may cause undesirable instability.